A radioimmunoassay method (radioimmunoassay (RIA) or immunoradiometric assay (IRMA)) has long been known as a quantitative immunoassay method. In this method, a competitive antigen or antibody is labeled with a radionuclide, and the antigen is quantitatively measured based on the measurement result of specific activity. That is, a target substance such as an antigen is labeled, and the target substance is indirectly measured. Because the radioimmunoassay method has high sensitivity, the method has greatly contributed to clinical diagnosis. However, because there are problems involved in handling radionuclides, a dedicated facility and apparatus are required when using the radioimmunoassay method.
Therefore, as a more workable method, for example, a method using labels such as fluorescent substances, enzymes, electrochemiluminescent molecules, or magnetic particles has been proposed. When using a fluorescent label, an enzyme label, an electrochemiluminescent label or the like as a label, detection of the target substance is performed by measuring the rate of absorption or transmittance of light or by measuring the light emission amount using an optical measurement method. Enzyme immunoassay (EIA) that uses enzymes as labels is a method in which, after carrying out an antigen-antibody reaction, enzyme-labeled antibody is caused to react, and a substrate for the enzyme is then added to cause color development, after which colorimetric determination is performed based on the absorbance.
Further, several research institutions have reported biosensors that use a magnetic sensor element to indirectly detect a biomolecular using a magnetic substance as a label. Various types of magnetic sensor elements can be used for this type of detection. Non-Patent Document 1 (H. A. Ferreira, et al, J. Appl. Phys., 93 7281 (2003)) describes a detection method using a magnetoresistance effect element. Non-Patent Document 2 (Pierre-A. Besse, et al, Appl. Phys. Lett. 80 4199 (2002)) describes a detection method using a Hall element. Non-Patent Document 3 (Seung Kyun Lee, et al, Appl. Phys. Lett. 81 3094 (2002)) describes a detection method using a Josephson element. Non-Patent Document 4 (Richard Luxton, et al, Anal. Chem. 16 1127 (2001)) describes a detection method using a coil. Non-Patent Document 5 (Horia Chiriac, et al, J. Magn. Magn. Mat. 293 671 (2005)) describes a detection method using an MI element (magneto-impedance element) in which the magnetoimpedance varies.
The detection methods for a biological substance using such elements have different characteristics. In particular, the MI element advantageously has a simple structure.